A superscalar processing system includes multiple execution units for simultaneously executing multiple instructions. In some processing systems, instructions are executable out-of-order relative to their programmed sequence within the instruction stream. Nevertheless, one or more selected instructions might be subject to execution serialization, where the execution serialized instruction is executable only in-order of its programmed sequence. Also, some of these processing systems are designed to write instructions' results to storage locations (e.g. architectural registers) in-order of the instructions' programmed sequence. According to typical previous techniques, such in-order writeback adversely impacts the time for instructions to be completed by the processing system. Moreover, such in-order writeback adversely impacts the time for an execution serialized instruction to be executed by the processing system.
Thus, a need has arisen for a processing system and method of operation, in which in-order writeback less adversely impacts the time for instructions to be completed by the processing system and for an execution serialized instruction to be executed by the processing system.